Frequency subtractors of the type discussed herein are frequently used with certain sensor circuitry which senses air pressure, for example, and develop two signal outputs. One output is typically a signal (F.sub.s) whose frequency varies in accordance with changes in the sensed variable (air pressure), and the other signal (F.sub.r) is a constant frequency, reference signal. These two signals are typically applied to a frequency subtractor (such as shown in FIG. 1) in the form of a flip-flop circuit whose output consists of a square-wave signal whose frequency is equal to the difference (F.sub.r -F.sub.s) in frequency between the signal inputs. That frequency difference is representative of the sensed variable and is usually processed for further use in accordance with the particular application. U.S. Pat. Nos. 4,392,382 and 4,550,611, assigned to the assignee of this invention, illustrate pressure sensor applications of such a frequency subtractor.
As discussed above, the heart of the typical frequency subtractor is a flip-flop circuit. In the case where a D-type flip-flop is used, the signals F.sub.s and F.sub.r are applied to the "D" and "clock" inputs, respectively, while the output F.sub.r -F.sub.s is developed at the Q output. This relatively simple approach to finding the difference between two frequencies is cost-effective and practical for many applications. However, the output signal representative of F.sub.r -F.sub.s contains an inherent uncertainty due to jitter caused when the flip-flop is clocked by F.sub.r at about the same time that the signal F.sub.s is undergoing a transition. This problem of jitter is discussed in more detail below. Suffice it to say at this point that the jitter in the output of such a frequency subtractor may account for a large fraction of the total error that the system can tolerate. In systems requiring more precision, the jitter must be substantially reduced.
Another factor to consider in precision systems is the amount of quantization error associated with the frequency subtractor. With the conventional type of subtractor shown in FIG. 1, the quantization error (discussed in more detail later) severely limits the speed with which the frequency subtractor can develop an accurate output, and it also contributes to jitter.